The present invention relates to disk drive circuits and, more particularly, to a system and method for improving frequency response of a preamplifier in a hard-disk drive.
A hard disk drive generally includes a stack of rotating disks or platters and a spindle motor that is controlled to cause the disks to rotate. Data is generally stored in the form of a sequence of magnetically polarized regions on the surface of the disk. The sequences, known as tracks, typically appear as concentric circles on the disk.
A magneto-resistive (xe2x80x9cMRxe2x80x9d) read/write head, which is sensitive to changes in magnetic flux, reads and/or writes data to a disk as it is supported by an arm above the surface of the disks in close proximity to the surface thereof. As a disk rotates under the read/write head, the read/write head xe2x80x9cfliesxe2x80x9d on a thin cushion of air created by the motion of the disk. The read/write head reads data from a disk by sensing flux changes (e.g., changes in polarity) on the magnetic surface of an associated disk as it passes beneath the read/write head. The flux change, in turn, causes a change in the resistance of the head. The MR read/write head provides a corresponding differential output signal to an associated differential MR preamplifier.
Background FIGS. 1 and 2 respectively illustrate two of several possible combinations of head bias (e.g., voltage or current source) and sense schemes, such as are commonly employed in hard-disk drive applications. In FIGS. 1 and 2, a resistor designated as Rmr represents the MR head. The head resistor Rmr is connected to the preamplifier through a trace-suspension assembly (xe2x80x9cTSAxe2x80x9d), which may be modeled by a transmission-line or an equivalent RLC network.
It is known that the value of Rmr for read/write heads varies widely for different manufacturers of read/write heads. In addition, read/write heads from a given manufacturer can vary significantly. Typical specification limits for Rmr range, for example, from about 30 xcexa9 to about 80 xcexa9.
The preamplifier has an input impedance (Zin). For a typical configuration, such as shown in FIGS. 1 or 2, Rmr and Zin form a voltage divider, which tends to attenuate the overall preamplifier gain by the factor:                                           Z            in                                              R              mr                        +                          Z              in                                      .                            Eq        .                  xe2x80x83                ⁢        1            
Consequently, more gain attenuation will result with this configuration as Rmr changes from 30 xcexa9 to 80 xcexa9.
Preamplifier designs are increasingly being implemented with xe2x80x9clowerxe2x80x9d input impedance Zin values. Because a lower Zin matches closer to a typical Rmr value, the preamplifier frequency response tends to be flatter. However, the gain attenuation will be larger than a higher-Zin design for the same Rmr range change (see, e.g., Eq. 1).
To illustrate the above, FIG. 3 shows frequency response curves 10, 12, 14 for different values of Rmr, namely for Rmr=35 xcexa9, Rmr=45 xcexa9, and Rmr=65 xcexa9, respectively, for a case of Zin=358 xcexa9. Also illustrated are curves 16, 18, and 20 for Rmr=35 xcexa9, Rmr=45 xcexa9, and Rmr=65 xcexa9, respectively, for a situation where Zin=80 xcexa9. FIG. 3 illustrates that the gain drops as Rmr increases. In addition, the range of gain change is greater for smaller values of Zin. The frequency-response shape also varies according to the Rmr values. AC performance appears better in the case of a lower value of Zin. In particular, the low value Zin results in less gain peaking or drooping, and less bandwidth variation over the Rmr range.
In order to help accommodate variations in Rmr, preamplifiers have been designed to allow customers (e.g., hard-disk drive manufacturers) to dial in a different gain setting in an effort accommodate changes in gain due to different values of Rmr. If the programmable gain range and resolution are sufficient, the mid-band gain can be adequately re-aligned. This approach, however, tends to result in a non-optimized frequency-response shape.
The present invention provides a system and method that is operative to enhance a frequency response for a preamplifier system. The preamplifier includes an amplifier stage having at least one feedback network. The feedback network has a feedback resistance, which may be adjusted to improve a frequency response of the preamplifier.
By way of illustration, a magneto-resistive read/write head of a hard-disk drive may be coupled to an input of the preamplifier system. The magneto-resistive head has a resistance value. According to one particular aspect of the present invention, the feedback resistance of the feedback network may be controlled as a function of the resistance of the magneto-resistive head, such that the frequency response of the preamplifier system is improved.
By way of further illustration, the control of the feedback resistance may be implemented as a programmable register or a metal-mask trimming option, such as may be part of an integrated circuit chip containing the preamplifier system.
Another aspect of the present invention provides a method for improving a frequency response of preamplifier system, such as may be utilized to receive a signal from a magneto-resistive head of a hard-disk drive. A resistance value of the magneto-resistive head is sensed and employed to select a feedback resistance value for the preamplifier system. The selected feedback resistance value, in turn, is utilized to program the preamplifier system accordingly. For example, the feedback resistance of the preamplifier may be programmed to a value proportional to the resistance value of the magneto-resistive head. As a result, the frequency response for the preamplifier may be substantially optimized.
To the accomplishment of the foregoing and related ends, certain illustrative aspects of the invention are described herein in connection with the following description and the annexed drawings. These aspects are indicative, however, of but a few of the various ways in which the principles of the invention may be employed and the present invention is intended to include all such aspects and their equivalents. Other advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.